1. Field of the Invention
The present invention relates to the field of vision correction and more particularly to methods of modifying the cornea to provide improved vision.
2. Background Information
There is a need for vision correction which does not rely on either glasses or contact lenses. In some cases this need is a result of the fact that glasses and contact lenses are incapable of correcting the vision errors exhibited by a particular eye. In other cases, this results from an occupational or other requirement that the patient have near perfect vision without the use of glasses and contact lenses or because of a desire of the patient to avoid the use of glasses or contact lenses for other reasons. A variety of surgical techniques have been suggested to providing vision correction without the use of contact lenses and glasses. A number of these techniques are described briefly in the background portion of U.S. Pat. No. 4,923,467 entitled "Apparatus and Process for Application and Adjustable Reprofiling of Synthetic Lenticules for Vision Correction" by Keith Thompson; that patent is incorporated herein by reference in its entirety. Another surgical technique, discussed in U.S. Pat. No. 4,923,467, is known as photorefractive keratectomy and involves the use of intense ultraviolet light to remove corneal material by ablation without apparent thermal or mechanical damage to adjacent tissue. This procedure may be carried out using an argon fluoride (ArF) excimer laser at a wavelength of 193 nm. This procedure is irreversible since the removed tissue cannot be replaced. In order to correct vision, this procedure requires removal of corneal material from the central, visually active portion of the cornea and thus carries attendant risks of irreversible damage.
U S. Pat. No. 4,923,467 is directed to a technique for providing a synthetic lenticule which is attached to the cornea as a means to provide vision correction--a procedure which avoids removal of any tissue in the visually active portion of the cornea. In accordance with U.S. Pat. No. 4,923,467 a synthetic lenticule of biocompatible material such as collagen is fabricated to fit the patient's cornea and to provide the refractive changes necessary for vision correction. Such lenticules are attached to the eye by forming a shallow groove in the cornea outside the visually active portion of the cornea, placing an appropriate adhesive in the groove and attaching the synthetic lenticule to the eye by placing the lenticule in the intended position with its periphery disposed in the groove and secured therein by the adhesive. A variety of other techniques may be used for securing the lenticule to the cornea at the groove. This technique has the advantage that since the adhesive is disposed in the groove, the lenticule may be removed if necessary without any affect on the visually active portion of the cornea.
In the event that the lenticule as initially fabricated and attached does not provide the desired degree of correction, or if the eye's characteristics change subsequently so that additional correction is required, the correction provided by the lenticule may be changed by laser ablation of the lenticule itself. This is considered a substantial improvement over the direct ablation of the cornea because the cornea is unaffected by this ablation. Consequently, in the event that the lenticule cannot be corrected by further ablation, the lenticule may be removed and replaced with a substitute lenticule without adverse affect on the central, optically active, portion of the cornea.
A potential disadvantage of this technique is the need to form the peripheral groove for the mounting of the lenticule, the need to accurately configure both the posterior and anterior surfaces of the lenticule to mate with the existing contour of the cornea in order to provide the desired optical correction and to provide a smooth transition between the lenticule and the Bowman's layer of the cornea. Such a smooth transition facilitates the regrowth of epithelial cells across the lenticule in order to re-establish the epithelium as an overlying protective layer which separates the cornea from the exterior environment.
A technique for modifying the configuration of the cornea which retains the advantages of U.S. Pat. No. 4,923,467 while further reducing risks and simplifying procedures is desirable.